A typical prior art friction-feed plotter employs a plurality of abrasive drive rollers mounted adjacent to a platen or like sheet-supporting surface. A plurality of pinch rollers are mounted above the drive rollers and movable into and out of engagement with a sheet material overlying the drive rollers. The pinch rollers press the sheet material into engagement with the abrasive drive rollers, and the abrasive drive rollers are rotatably driven to frictionally drive the sheet material in its lengthwise direction. A tool head comprising one or more plotting tools, such as a pen for drawing or printing lines or other graphic images on the sheet material, or a knife blade for cutting the sheet material, is mounted over the sheet-supporting surface and driven in the widthwise direction of the sheet material to plot by printing on and/or cutting the sheet material.
Friction-feed plotters of this type are employed for printing on and/or cutting numerous different types of sheet materials, such as vinyl and other types of polymeric sheets, paper and cardboard, and are used to make graphic products, such as signs, packaging and other visual displays. Commercially-available sheet materials are provided in various widths. In addition, a single piece of sheet material may be cut into any of an infinite number of different widths, thus leaving behind usable remnants or unused portions which likewise may be formed in any of an infinite number of different widths. For example, a relatively wide sheet may be initially used to print and cut a relatively narrow sign or other graphic product. Thus, a substantial amount of sheet may remain unused after the initial graphic product is prepared. Many of the sheet materials used for making signs, packaging and other visual displays are relatively expensive, and therefore it is desirable to minimize any waste by using the remnants or unused sheet portions to make additional graphic products. As a result, it is desirable for friction-feed plotters to handle sheets of virtually any width within a predetermined range of widths, in order to accommodate both the commercially-available sheet widths and any usable remnants or unused portions of such sheets.
Typical prior art plotters for accommodating sheet materials of different widths are shown in U.S. Pat. No. 5,163,675 to Sunohara ("the '675 patent"). One such prior art plotter employs a relatively long main drive roller mounted on one end of the sheet-supporting surface, and a relatively short sub-drive roller mounted on the opposite end of the sheet-supporting surface. A main pinch roller is mounted above, and movable laterally over the main drive roller for engaging the edge of a sheet overlying the main drive roller, and a sub-pinch roller is mounted above the sub-drive roller for engaging the edge of a sheet overlying the sub-drive roller. The main drive roller is sufficiently long so that the opposing edges of a sheet of any desired width within a premeditated range of widths may be engaged between the main and sub-drive rollers and the main and sub-pinch rollers, respectively.
The '675 patent states that the length of the main drive roller must be relatively great in order to accommodate the desired range of sheet widths. The '675 patent further states that the main drive roller must be capable of uniformly contacting and holding the sheet material without unequal contact pressures along its entire length. Accordingly, the '675 patent states that the main drive roller must be manufactured with very high precision, and therefore is unsuitable for mass production.
In order to overcome the problems associated with the above-described prior art plotter, the '675 patent teaches the use of a drive roller group consisting of a single relatively long main drive roller, and a three relatively short sub-drive rollers serially arranged at predetermined spacings from the main drive roller. A first pinch roller is mounted over the main drive roller and is movable laterally over the main drive roller, and a second pinch roller is mounted over the sub-drive rollers and is movable laterally over the sub-drive rollers for accommodating sheets of different widths. Although the main drive roller of the '675 patent is longer than each of the sub-drive rollers, the plurality of sub-drive rollers, and their predetermined spacings from the main drive roller, permit the main drive roller to be shorter than its counterpart in the above-described prior art plotter.
One of the drawbacks of the plotter illustrated in the '675 patent is that it still employs one relatively longer main drive roller in order to accommodate a sufficient range of sheet widths. Alternatively, if the longer main drive roller were eliminated, and the main and sub-drive rollers were of the same width, one or more additional sub-drive rollers would have to be added in order to accommodate the same range of sheet widths. The abrasive drive rollers employed in plotters of this type can be relatively expensive, however. Thus, the cost of the abrasive drive rollers in a plotter of this type can be substantially greater than in plotters employing a fewer number of such rollers.
Another problem encountered with prior art friction-feed plotters is that the sheet material may be unevenly driven by the abrasive drive rollers, and in turn assume a skewed or cocked position. This type of error is referred to as skew error. One approach for correcting skew error employed by the Assignee of the present invention is to independently drive the abrasive drive rollers engaging the opposing sides of the sheet material, and to provide at least one sensor for detecting any lateral deviation of the sheet material from its feed path. If a lateral deviation is detected, the abrasive drive rollers are controlled to rotate at different speeds to correct the lateral error.
The '675 patent teaches that the plurality of abrasive drive rollers should be rotated synchronously at the same peripheral speeds. In addition, the plotter of the '675 patent does not lend itself to driving the abrasive rollers independently of each other, or driving groups of the abrasive rollers independently of each other. Thus, the '675 patent does not explicitly recognize, nor does the plotter of this patent otherwise lend itself to correcting skew error.
Accordingly, it is an object of the present invention to provide friction-feed plotters which overcome one or more of the above-described drawbacks and disadvantages of the prior art.